This invention relates to an electrically insulated cable having a sheath that is heat-fusible to polyamide or polyester resin, and a connecting arrangement that makes it possible to form a highly reliable waterproof joint between the abovementioned cable and a housing made of polyamide or polyester resin at a low cost.
With the advancement in car electronics, today's cars are equipped with various control systems including antilock brake systems (ABS). Most of such control systems comprise sensors for converting physical quantities such as temperature, speed, pressure, etc. into electric signals, an ECU (electric control unit) for processing signals from the sensors, and actuators controlled by the signals from the ECU. Sensors for an ABS are wheel rotation speed sensors provided near the vehicle wheels for converting wheel speed information into electric signals, which are inputted in the ECU. The ECU processes the signals inputted and produces electric output signals, which are transmitted through cables to actuators to activate them.
Such wheel speed sensors include electromagnetic pickup sensors and Hall-effect sensors. Since wheel speed sensors are exposed to water while the vehicle is traveling, not only the sensors themselves but the joints between the sensors and cables have to be sufficiently water-resistant.
The housing for a wheel speed sensor is typically made from engineering plastics such as PBT (polybutylene terephthalate) resin, 6-nylon resin, 6, 6-nylon resin, and 6T-nylon (aromatic nylon) for higher dimensional accuracy and mechanical strength, and better moldability.
As a cable (sensor cable) for connecting each wheel speed sensor to the ECU, one shown in FIG. 2 is used which comprises insulated wires a having a conductor covered with an insulator made of polyvinyl chloride or flame-retardant polyethylene, and a sheath b covering the insulated wires a, or one shown in FIG. 3, in which an intermediate layer c is provided between the insulated wires a and the sheath b. The sheath of such a cable A is ordinarily made from a crosslinked thermoplastic polyurethane elastomer composition for higher flexibility, abrasion resistance, bend resistance, water resistance, etc. Such a sheath cannot be heat-fused to the abovementioned engineering plastics, so that the sensor cable joint portion has to be waterproofed with a separate seal member.
FIG. 4 shows a conventional joint between a wheel speed sensor and a cable. As shown, the sensor and the cable are connected together by injection-molding the housing H with a seal member B such as an O-ring fitted around the cable A. This joint structure is expensive because it needs an extra seal member such as an O-ring and an extra step of fitting the seal member around the cable.
As a solution to this problem, in a conventional arrangement (disclosed in Japanese patent application 7-194480), a sensor housing is injection-molded from PBT resin. During injection molding, the housing is heat-fused to a cable having its sheath made from a crosslinked thermoplastic polyester elastomer to waterproof the joint portion.
But a cable sheath made from a crosslinked thermoplastic polyester elastomer is not heat-fusible to a housing made from polyamide engineering plastics such as 6-nylon, 6, 6-nylon, etc. Thus, the joint portion therebetween cannot be waterproofed sufficiently. As a result of long-term endurance test, it has been found out that if a housing made from polyester engineering plastics such as polybutylene terephthalate (PBT) resin is used, too, water resistance tends to drop.
An object of this invention is to provide a cable having a sheath made of a material which is heat-fusible to a member made of polyamide engineering plastics so that the interface therebetween can be sealed at a low cost.
Another object of the present invention is to provide a cable which has an improved long-term waterproofness to a housing made of polyester engineering plastics.